Washington (CNN) -- Retired Supreme Court Justice John Paul Stevens has voiced support for the killing of al Qaeda terrorist leader Osama bin Laden by U.S. forces, saying it was legally justified.

In remarks Thursday evening at his alma mater, Northwestern University, the 91-year-old former justice said the order by President Barack Obama for the covert mission by U.S. Navy SEALs was "to remove an enemy who had been trying every day to attack the United States," according to two people who attended a symposium and dinner that was closed to the media.

Stevens said he was pleased the president took the risky decision to launch the May 2 commando assault on bin Laden's compound in Pakistan. The justice added, "I must say I was very proud of the SEALs."

No sitting member of the Supreme Court has commented on the bin Laden killing and is not likely to, since current or related executive branch issues may someday come before them.

There have been legal questions surrounding whether U.S. and international law would permit a unilateral executive decision to kill a terrorist leader with no ties to any government. Attorney General Eric Holder said the day after the mission, "It's lawful to target an enemy commander in the field."

As founder and head of al Qaeda, bin Laden was viewed by the administration as a combatant actively involved in past and current hostilities against the United States and other countries.

The insistence by Obama officials that the killing was justified come despite bin Laden not being armed when commandos stormed his third-floor room. Those officials insist the Saudi native "resisted" and made no clear indication he would surrender.

Stevens said based on his knowledge of the facts, "I haven't the slightest doubt it was entirely appropriate for American forces to act" as they did. "It was not merely to do justice and avenge September 11."

The Wall Street Journal Law Blog first reported Stevens' remarks.

He stepped down from the high court last year after nearly 35 years on the bench, and was replaced by Justice Elena Kagan.

Unusually for a retired justice, he has been outspoken on current hot-button cases his former colleagues are considering and on other news events. He voiced support in November for a planned Islamic community center near the site of the September 11 terrorist attacks in New York, urging religious and ethnic tolerance.

He announced this week he has almost completed writing a memoir of sorts, a look at the five chief justices he has known and worked with over the past 64 years. Called the "Five Chiefs," and to be published by Little Brown this October, the book will also give an inside look at the court and Stevens' personal insights on colleagues past and present, including current Chief Justice John Roberts.

Before retiring, Stevens had been a consistent supporter of limited rights for terror suspects in U.S. custody overseas to challenge their detention in federal courts. In the first terrorism case to arise in the high court from the 9/11 attacks, Stevens said foreign-born terror suspects captured abroad and held at a Navy-run prison at Guantánamo Bay, Cuba, did have some basic rights.

"What is presently at stake is only whether the federal courts have jurisdiction to determine the legality of the executive's potentially indefinite detention of individuals who claim to be wholly innocent of wrongdoing," he wrote in 2004.

Stevens is a World War II veteran. He served as an intelligence officer in Hawaii, where his work included supervising the cracking of secret Japanese codes. He was speaking in public Friday to graduates at his old law school in Chicago, where he graduated in 1947 at the top of his class.